A thermal printer typically contains one or more thermal printing heads that comprise linear arrays of heating elements (also referred to herein as “printing head elements”). The thermal printing head is usually arranged to span the width of an output medium onto which an image is to be printed. The output medium may be an individual sheet or a continuous web comprising paper, plastic, or similar material. During printing, the output medium is transported perpendicular to the direction spanned by the thermal printing head while the printing head elements are energized in a pattern corresponding to an image. The printed image on the output medium arises from differential heating by the printing head elements. Each of the printing head elements (which may number in the hundreds per inch), when activated, forms color on the portion of the output medium passing underneath the print head element, creating a spot having a particular density. Regions with larger or denser spots are perceived as darker than regions with smaller or less dense spots. Digital images are rendered as two-dimensional arrays of very small and closely-spaced spots.
The density of the output produced by the thermal printing head element is a function of the amount of energy provided to the printing head element. The amount of energy provided to the printing head element may be varied by, for example, varying the amount of power provided within a particular time interval or by providing constant power for a longer or shorter time interval.
The mechanism of color formation on the output medium may involve transfer of a dye or pigment from a donor to a receiver sheet, or may occur through activation of color-forming chemistry in the output medium itself. The former is commonly referred to as “thermal transfer printing” and the latter as “direct thermal printing”.
In International Application No. PCT/US02/15868 (which corresponds to U.S. patent application Ser. No. 10/151,432, cross-referenced above), entitled “Thermal Imaging System,” there is described a direct thermal imaging system in which one or more thermal printing heads can write more than one color in a single pass on a single output medium by selectively activating color-forming chemistry in particular layers of the output medium. A printer using the pulsing techniques and output medium described in International Application No. PCT/US02/15868 can address three image-forming layers of the output medium at least partially independently from the same surface of the output medium. In this way, a full-color image in three subtractive primary colors (cyan, magenta and yellow) may be printed in a single pass onto the output medium without the need for dye donor elements, inks, etc.
Because the printer that addresses such an output medium consists essentially only of a thermal printing head, control electronics, a transport mechanism and a power supply, it has become possible to conceive of smaller printing devices than have been achievable with other printing techniques.
There are, however, situations in which it may be required to provide bulk heating or cooling of a thermal printing head in a printer, and such bulk heating or cooling may be incompatible with a requirement for a printer of small size if conventional approaches are used. The terms “bulk heating” and “bulk cooling” of the thermal printing head, as used herein, refer to such heating and cooling as affects the average temperature of the entire thermal printing head, as opposed to the heating of the printing head elements when forming an image, in which the heating is localized to the immediate vicinity of the heating element.
Bulk heating or cooling of the thermal printing head may be required for a number of reasons. For example, in the current state of the art, the efficiency with which heat is transferred into an output medium from a thermal printing head is fairly low, with the result that during the printing of an image, the thermal printing head itself is heated. It may become so hot that image discrimination may be affected, and cooling may therefore be required.
At the other extreme, the temperature of the thermal printing head may be lower than the desired operating temperature at the start of printing (especially outdoors in winter), and some pre-heating may be required before printing can commence.
In prior art printers, cooling of the thermal printing head has typically been achieved by use of a heat sink (i.e., a large thermal mass) in thermal contact with the thermal printing head. The heat sink itself is typically cooled by, for example, air or a circulating liquid. Fans are often used to assist air cooling. Heat pipes or other means of high thermal conductivity may be used to transfer heat to/from a location where cooling can be most efficient.
When heating of the thermal printing head is required, this may be achieved by activation of the printing head elements themselves, or alternatively by some other means of heating of the heat sink. Unfortunately, because the heat sink and the rest of the thermal printing head are in the thermal contact, any preheating means must provide enough energy to raise the temperature of the entire assembly.
When it is required to minimize the size of the printer, these prior art methods for cooling or heating may be inadequate. There may be insufficient space to provide for air flow or liquid for cooling. In particular, there may be no practical way to incorporate a fan into a small printer. Moreover, when the printer is intended for battery-powered operation with a minimum size, it is clearly necessary to minimize the power that must be drawn from the battery. When it is necessary to preheat the thermal printing head, therefore, it would be preferred that a mass required to be heated be as small as possible. This requirement is in conflict with the requirement for a heat sink with a large thermal mass for cooling during printing.
Even when size is not a constraint in the design of a thermal printer, it still may be desired to have additional flexibility in the heating or cooling of the thermal printing head.